Tuberculosis is currently an enormous health problem in the United States and worldwide especially among the elderly and immunocompromised patients, Three million people die from this disease annually, a death rate that ranks among the highest from any infectious cause. There is no effective vaccine and antimicrobial therapy is becoming less effective, in part because of an inadequate understanding of the pathogenesis of tuberculosis and the immunobiology of the causative agent, Mycobacterium tuberculosis (M.tb). M.tb is an intracellular bacterium that multiplies within circulating monocytes and mature tissue macrophages. Cell mediated immunity is essential in host defense against M.tb and other intracellular pathogens. Therefore, the mononuclear phagocyte plays a central role in pathogenesis by serving both as a reservoir for M.tb and as an antigen-presenting cell. We recently determined that phagocytosis of M.tb by human monocytes is mediated by complement receptors (CR) on the monocyte surface and complement component C3 bound to the surface of the organism. These studies have laid the foundation for further studies aimed at enhancing our understanding of the role of complement and macrophage CR in M.tb intracellular biology. We plan to utilize our in vitro assay of mycobacterial invasion to identify human macrophage receptors that mediate M.rb phagocytosis and to determine whether molecules known to regulate CR play a role in M.tb phagocytosis. We will also determine how phagocytosis of M.tb by macrophages influences the surface expression and synthesis of CR and a related macrophage surface molecule, LFA-1. We will use flow cytometry, ELISA, and Northern blots for these studies. We will also determine the dominant C3 acceptor molecule on the M.tb surface using immunologic techniques. Once we have identified this molecule, we will then explore the location of C3, CR, and this acceptor molecule in macrophages using electron microscopy and will determine the role of C3 and the acceptor molecule in macrophage-dependent lymphocyte proliferation. We will compare virulent and attenuated strains of M.tb in these studies to identify and characterize important virulence factors. These proposed studies will expand our knowledge of the M.tb-mononuclear phagocyte interaction and should provide new strategies for the prevention and treatment of tuberculosis and other diseases caused by intracellular pathogens.